As a structure of a submarine cable having optical fibers as transmission lines, for example, ones of the structures shown in FIG. 1 or FIG. 2 have been proposed.
These structures will be explained below. 1 is a bundle of optical fibers obtained by twisting together a plurality of optical fibers 1a or such a bundle buried in an ultraviolet curing synthetic resin (ultraviolet curing urethane) or such a bundle buried in a thermoplastic synthetic resin with a tension-bearing member 1b passed through the center of the optical fibers. 2 is a pressure-proof layer for protecting the optical fiber unit 1 from water pressure, while 3 is a tension-bearing layer mainly formed by twisting together steel wire (piano wire) so as to be able to handle the tension applied to the cable.
This tension-bearing member layer 3 is made a single-layer or multiple-layer structure, has a tension-bearing ability enabling it to withstand the tensile load due to the weight of the cable itself at the time of laying or retrieving the cable, and acts to protect the cable from outside damage.
4 is a metal layer air-tight with the bundle of the tension-bearing member layer 3 and forming a power feed conduit to a repeater. Normally, it is comprised of a metal tape made of copper, aluminum, etc. attached longitudinally, welded together, and reduced in diameter (shrunk) to form it into a tube.
Further, 5 and 6 are insulating layers (sheaths) meant for insulation from the seawater and formed by low density and high density polyethylene etc.
Among these cables, the one shown in FIG. 1 uses a combination of three approximately fan-shaped deformed wires as the pressure-proof layer 2. Further, in FIG. 2, the tension-bearing member layer 3 is configured to form a pressure-proof shell by the interaction of the tension-bearing wires wound in two layers.
The tension-bearing ability of the submarine optical cable is mainly provided by the pressure-proof layer 2 and the tension-bearing member layer 3. The tensile strength of the steel wire (piano wire) used for the tension-bearing members 3 is of a level of 2200 MPa. On the other hand, for the approximately fan-shaped deformed wire used for the pressure-proof layer 2, as high strength deformed wire for submarine optical cable using for example wire rod for long, high tension steel wire superior in weldability and cold workability, Japanese Examined Patent Publication (Kokoku) No. 7-65142 proposes deformed wire with an approximately fan-shaped cross-section having a tensile strength of at least 1226 MPa produced from steel wire defined as Ceq=C+(Mn+Cr)/5≧0.57%. However, the maximum value of the tensile strength achieved is on the level of 1520 MPa. This is currently lower than the tensile strength of piano wire.
In recent years, an increased communication capacity has been demanded from submarine cable systems. To meet with the increase in communication capacity, higher performance of optical fibers and an increased number of optical fibers accommodated in submarine optical cable have been demanded.
Along with the increase in the number of optical fibers accommodated, the optical fiber units have increased in outside diameter. Therefore, the inside diameter of the pressure-proof layer 2 has become greater. To prevent the cable outside diameter from increasing along with this, the thickness of the pressure-proof layer 2 has to be made smaller, so the tension-bearing ability of the cable drops. If the tension-bearing ability drops, since the tension-bearing ability is designed to handle the tensile load due to the weight of the cable itself at the time laying or retrieving the cable, there is the problem that the tension-bearing ability of the cable has to be kept from being exceeded by making the depth of use of the cable shallower.
On the other hand, if not changing the thickness of the pressure-proof layer 2, the outside diameter of the pressure-proof layer 2 becomes larger. In this case, there is the problem that along with the increase in the outside diameter of the pressure-proof layer 2, the tension-bearing ability of the cable has to be kept from being exceeded by making the depth of use of the cable shallower.
Further, along with an increase in the communication capacity and an increase in the number of fibers, the processing capability and number of amps required in the repeaters increase and the amount of power supplied to the repeaters increase. A repeater is supplied with power from a station on land through the metal layer 4 serving as the power feed conduit. Along with an increase in the amount of fed power, the voltage applied at the station also becomes high, so a reduction in the conduction resistance of the metal layer 4 forming the power feed conduit is required. To reduce the conductor resistance of the metal layer 4, it is necessary to increase the cross-sectional area of the metal layer 4. This means an increase in the thickness of the metal layer 4, so the weight of the cable increases.
To solve the problem of an increase in the weight of the cable or a decline in the tension-bearing ability being accompanied with the depth of use of the cable becoming shallower, the tension-bearing members of the cable have to be raised in strength.